Method of producing substantially spherical magneto-plumbite ferrite particles

ABSTRACT

Substantially spherical magneto-plumbite ferrite (barium or strontium ferrite) particles are formed from well-dispersed ultra-fine substantially spherical iron-based oxide and/or hydroxide particles as precursor particles. The precursor particles are mixed with a colloidal barium or strontium carbonate (BaCO 3  or SrCO 3 ), and with small amounts of a byproduct, such as sodium or potassium chloride (NaCl or KCl) or hydroxide (NaOH or KOH) or nitrate (NaNO 3  or KNO 3 ), functioning as a flux to lower the calcination temperature. The particles are filtered out of the mixture, dried, and calcined for a time sufficiently long and/or at a temperature sufficiently high to form magneto-plumbite ferrite from the precursor particles, and for a time sufficiently short and/or a temperature sufficiently low to maintain the general spherical shape of the precursor particles. The particles are used for forming a magnetic recording media by dispersing the particles in a magnetic paint and coating the paint onto a substrate, or by dispersing the particles in a self-supporting material.

FIELD OF THE INVENTION

This invention relates to substantially spherical magneto-plumbiteferrite (barium or strontium ferrite) particles and methods for theirproduction, and to magnetic recording media formed therewith.

BACKGROUND OF THE INVENTION

Magnetic recording media are employed in a wide variety of applications,including identity cards, credit cards, banking cards, parking permits,hotel key cards, tollgate cards, data tapes, and floppy disks. In theseand similar applications, it is desirable to provide a recording mediumthat both minimizes unintended erasure and maximizes storage capacity(bit density).

A magnetic recording media requires adequately high coercivity (ameasure of magnetic field strength of a magnetized substance and of itsresistance to demagnetization) to minimize accidental loss of storeddata. In magnetic stripe cards, for example, accidental erasuresassociated with low coercivity of the recording medium account for 60%of card failures.

A magnetic media also preferably has relatively low magnetic interaction(a measure of the degree of magnetic interaction of one point on themedia with adjacent areas on the media). Lower magnetic interactionallows increased bit density, providing increased data storage capacityin the same space.

Increases in storage capacity could provide extended capabilities indevices employing magnetic media. In the case of magnetic stripe cards,because a standard banking credit card has only 140 bytes of storage, atypical consumer uses several separate magnetic stripe cards. Largerstorage capacity would allow the combination of several cards into onemultifunctional magnetic card. New functions not currently performed bymagnetic cards could even be added to such a card.

Magneto-plumbite ferrite particles have been used in magnetic recordingmedia. Magneto-plumbite ferrite particles have the advantage ofrelatively high coercivity (e.g., anywhere from 300 to more than 3500Oe), and have shown a corresponding greater resistance to erasure thanmedia using typical acicular (needle-like) metal particles.

But magneto-plumbite ferrite particles have the disadvantage of tendingto group together in clumps or stacks during general production,processing, and handling of the particles. Additionally, in theproduction of a magnetic layer employing magneto-plumbite ferriteparticles, the still-wet layer is subjected to a magnetic field toorient the particles. This magnetic orientation process significantlyincreases particle grouping of magneto-plumbite ferrite particles.Groups of particles tends to act magnetically as a single particle,resulting in uneven and larger-than-desired effective particle sizes andincreased magnetic interaction. Such groups of particles produce unevenmagnetic properties in a magnetic media, resulting in high media noiseand correspondingly lower maximum bit densities. To increase the storagecapacity of a magnetic media, the density of such defects must bedecreased in order to maintain an adequate media signal-to-noise ratio.

Grouping of particles during particle preparation can result in a smallamount of strongly bonded clumps or stacks (agglomerates) remaining inthe prepared powder. Such agglomerates do not disperse during themilling process for preparation of a magnetic paint. After the millingprocess, these non-dispersed particles clog filter pores during thefiltering process, decreasing filter efficiency. The non-dispersedparticles can also contribute to defects such as pinhole, stain, andrough surface on the top surface of the magnetic layer. These defectscause various problems, such as media noise (including dropout andsignal spike), collision between head and media, etc.

At CardTech/SecurTech in 1996, Eastman Kodak Company announced theIntelligent Magnetics card system, having comparable capabilities to achip (or smart) card but at much less cost. The MR (magneto resistive)head used for high-density storage applications such as the IntelligentMagnetics card system is much more sensitive than inductive heads usedin current card systems. Consequently, a magnetic card for such a systemrequires a small concentration of magnetic particles uniformly dispersedthroughout the whole card. If there are clumps or stacks ofnon-dispersed magnetic particles in a card, a magnetic field higher thanthat of the surrounding magnetic particles will be generated, resultingin a signal spike. In a card designed for high storage capacity,uniformity of dispersion is critical to obtaining a stable signal fromthe card.

For all of the above reasons, it is desirable to minimize particlegrouping and maximize the uniformity of dispersion of magneto-plumbiteferrite particles for use in magnetic recording media.

SUMMARY OF THE INVENTION

The present invention introduces substantially sphericalmagneto-plumbite ferrite particles, processes for the production of suchparticles, and magnetic media employing such particles. The particles ofthe present invention provide improved uniformity of dispersion anddecreased magnetic interaction between particles in magnetic recordingmedia compared to previous forms of magneto-plumbite ferrite particles.The process for the preparation of the substantially sphericalmagneto-plumbite ferrite particles results in a narrow particle sizedistribution, controllable coercivity (H_(c)), and excellent dispersionstability with reduced particle stacking or clumping in the organicsolvents and binders used to produce magnetic recording media.

Particle clumping and stacking in media is influenced by severalfactors, including properties of the particles such as particle shape,diameter, and aspect ratio, and other factors such as viscosity andcomposition of the magnetic paint, the applied magnetic field used forparticle orientation, and milling conditions during processing of theparticles. However, it has been found that a principle cause of particlestacking in magneto-plumbite ferrite particles is the platelet shape oftypical magneto-plumbite ferrite particles. Typical magneto-plumbiteferrite particles have a flat, hexagonal shape like a six-sided plate.The mean aspect ratio of such “platelet” magneto-plumbite ferriteparticles, i.e., the mean ratio of the longest dimension (diameter) tothe shortest dimension (thickness), is typically not less than about3:1, and can be significantly higher. Particles having higher aspectratios tend to stack or clump together more than those having loweraspect ratios.

Acicular (needle-like) magneto-plumbite ferrite particles have also beenreported. Acicular particles, with aspect ratios often as great as 10:1or more, are also difficult to disperse evenly.

The substantially spherical shape and the associated low aspect ratio(approaching 1:1) of the particles of the present inventionsignificantly reduces the tendency of the particles to form stacks orclumps, as compared to previous magneto-plumbite ferrite particles.Preventing particles from clumping or stacking decreases the degree ofmagnetic interaction and improves dispersion stability in a magneticpaint or other magnetic medium. The even dispersion that is achievedwith the use of the particles of the present invention allows thecreation of higher-density magnetic media, allowing more information tobe stored within the same amount of space. The increased storagecapacity allows the creation of such devices as magnetic cards thatfunction like smart cards, and higher capacity magnetic storage disksand tapes for digital data handling and storage.

The chemical formula of the particles of the present invention is asfollows:

AO.n(Fe_(2-x)M_(x)O₃)  (1)

where A is Ba, Sr, or mixtures thereof, n is within the range of fromabout 5.0 to about 6.0, and M is more than one of the group ofsubstitution elements Co, Zn, Ni, Mn, Al, Ti, Sn, Si, Nb, and Ta, and xis within the range of from about 0 to about 0.35. The particles aresubstantially spherical, having a mean aspect ratio generally less thanabout 2:1, desirably less than about 1.5:1, most desirably less thanabout 1.25:1.

The particles of the present invention are prepared by mixing ultra-fine(e.g., about 0.05 to about 1.2 μm diameter) substantially sphericaliron-based oxide and/or hydroxide particles, such as magnetite (Fe₃O₄),maghemite (γ-Fe₂O₃), hematite (α-Fe₂O₃), spinel ferrite ((Co, Ni, Zn,Mn, Ti, Si)_(x)Fe_(3-x)O₄), and/or iron hydroxide (FeOOH) with a sourceof Barium and/or Strontium, such as colloidal barium or strontiumcarbonate (BaCO₃ or SrCO₃), with small amounts of a byproduct, such assodium or potassium chloride (NaCl or KCl), hydroxide (NaOH or KOH), ornitrate (NaNO₃ or KNO₃) functioning as a flux to lower the calcinationtemperature.

The solid phase of the dispersion is then filtered off, dried, andcalcined at a temperature sufficiently high and for a time sufficientlylong to produce the desired magneto-plumbite ferrite, yet sufficientlylow and sufficiently short to allow the particles to substantiallyretain the shape of the iron-based oxide/hydroxide precursor particles.Temperatures generally not more than about 900° C. allow the particlesto substantially retain the shape of the precursor particles, desirablyin the range of about 730° C. to about 870° C. In the working examplesdescribed below, particles according to the present invention wereproduced at calcination temperatures in the range of about 780° C. toabout 810° C., at times from about 1 to about 2 hours.

The substantially spherical magneto-plumbite ferrite particles of thepresent invention prepared as described herein have a coercivity in therange of from about 1000 Oe to about 5500 Oe, high saturationmagnetization above about 35 emu/g, and narrow coercivity distribution(evidenced by a measured switching field distribution (SFD) of 0.40 forspherical barium ferrite particles compared to 0.60 for platelet bariumferrite particles in an otherwise identically-prepared magnetic medium),while maintaining excellent dispersibility and reducing or eliminatingparticle stacking. These properties are well suited to magneticrecording applications. Hand-coated magnetic tapes prepared with theparticles of the present invention have shown good magnetic properties,as set forth in the examples below. Thus the substantially sphericalmagneto-plumbite ferrite particles of the present invention may suitablybe used as the magnetic powder material of magnetic recording tape, andwould also find application in such media as magnetic stripe cards,floppy disks, etc., particularly for high-capacity storage applications.

The process of the present invention economically allows the massproduction on an industrial scale of the spherical magneto-plumbiteferrite particles having the desired properties mentioned above.Magnetic storage media may be prepared from the particles of the presentinvention by conventional means, such as mixing the particles in amagnetic paint, then coating the paint on a substrate such as a card, atape, or a disk, or such as mixing the particles directly into a polymeror the like, then forming a card, a tape or a disk of the polymer.

Additional objects, features, and advantages of the invention willbecome more apparent from the following detailed description andaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-4 are TEM (transmission electron microscope) micrographs ofsubstantially spherical magneto-plumbite ferrite particles of thepresent invention.

FIG. 5 is a graph of the measured value of positive ΔM as function ofthe measured orientation ratio (OR) in hand-coated magnetic tapesprepared from spherical barium ferrite powder and from platelet bariumferrite powder.

FIG. 6 is a side view of a magnetic recording tape made in accordancewith the present invention.

FIG. 7 is a side view of a magnetic recording medium having magneticparticles dispersed throughout the medium made in accordance with thepresent invention.

FIGS. 8 and 9 are computer-generated plots of the results of X-raydiffraction characterization of particles of the present inventionproduced by the methods of the present invention.

FIG. 10 (PRIOR ART) is a TEM micrograph of prior-art plateletmagneto-plumbite ferrite (barium ferrite) particles.

DETAILED DESCRIPTION

The present invention includes substantially spherical magneto-plumbiteferrite particles, magnetic media formed of such, and processes forproducing such. The chemical formula of the particles is as given aboveby formula (1). The shape of the particles is substantially spherical,as opposed to the platelet and acicular magneto-plumbite ferriteparticles previously known. The particles of the invention have a meanaspect ratio of less than about 2:1, desirably less than about 1.5:1,and more desirably less than about 1.25:1, as contrasted with aspectratios of about 3:1 and greater for previously known magneto-plumbiteferrite particles.

In brief, the process for making substantially sphericalmagneto-plumbite ferrite particles comprises providing a well-dispersedultra-fine (e.g., from about 0.05 to about 1.2 μm diameter)substantially spherical iron-based oxide and/or hydroxide particles,such as magnetite (Fe₃O₄), maghemite (γ-Fe₂O₃), hematite (α-Fe₂O₃),spinel ferrite ((Co, Ni, Zn, Mn, Ti, Si)_(x)Fe_(3-x)O₄), iron hydroxide(FeOOH), materials that form such on heating, and mixtures thereof. Theiron-based oxide and/or hydroxide particles (or sources thereof) aremixed with a colloidal source of barium and/or strontium such as bariumor strontium carbonate (BaCO₃ or SrCO₃) or mixtures thereof, with smallamounts of a flux agent. The flux agent can be a byproduct, such asGroup I metal salts including sodium or potassium chloride (NaCl orKCl), hydroxide (NaOH or KOH), or nitrate (NaNO₃ or KNO₃). The fluxagent lowers the calcination temperature.

In the particular examples to be discussed below, a well-dispersedaqueous slurry of spherical iron-based oxide and/or hydroxide, such asmagnetite (Fe₃O₄), maghemite (γ-Fe₂O₃), hematite (α-Fe₂O₃), spinalferrite ((Co, Ni, Zn, MN)_(x)Fe_(3-x)O₄), and iron hydroxide (FeOOH) wasprovided. This slurry was mixed with an aqueous source of metal A, suchas a chloride or nitrate or hydroxide solution, an aqueous source ofmetal M such as a chloride and/or nitrate solution (with A and M asdefined above in formula (1)), an aqueous sodium carbonate or potassiumcarbonate ((Na₂CO₃ or K₂CO₃) solution, and an aqueous sodium hydroxideor potassium hydroxide ((NaOH or KOH) solution. The resulting mixturewas then filtered and dried, and the particles thus obtained weresubjected to calcination at temperatures in the range from about 780° C.to about 810° C., which is within a desirable range of from about 730°C. to about 870° C. in any event desirably less than about 900° C. Aftercalcination, the agglomerated particles were wet-milled, filtered,washed, and dried.

The spherical magneto-plumbite ferrite particles in the particularexamples discussed below were characterized by a substantially sphericalparticle shape observed using a Transmission Electron Microscope (TEM).FIGS. 1 and 2 are TEM micrographs of such particles at 30,000 timesmagnification showing the substantially spherical shape of theparticles. FIG. 3 is a micrograph of such particles at 60,000 timesmagnification, and FIG. 4 is a micrograph at 120,000 timesmagnification, both showing the substantially spherical shape. This maybe contrasted with the shape of known (platelet) magneto-plumbiteferrite particles, as shown in FIG. 10 (prior art). FIG. 10 is a TEMmicrograph such known particles at 25,000 times magnification. Theparticles in the micrograph of FIG. 10 are oriented with plate facesperpendicular to the plane of the micrograph, so the hexagonal shape isnot evident, however, the relatively high mean aspect ratio of theparticles may thus be observed.

Magnetic properties of both magnetic particles and magnetic tapesprepared therefrom were measured, using a Vibrating Sample Magnetometer(VSM). Dispersion of the magneto-plumbite particles in hand-coatedmagnetic tapes was also evaluated by measuring ΔM defined as follows:

ΔM(H) (M _(D)(H)−(1-2M _(R)(H)))  (2)

where M_(D) is a normalized remanent magnetization from the dcdemagnetization (DCD) curve, and MR is a normalized temanentmagnetization from the isothermal remanent magnetization (IRM) curve.Particle stacking shows a high positive value of ΔM, which indicatesstrong positive magnetic interaction resulting in high media noise. Thevalue of positive ΔM may thus be used as an indication of particlestacking of magneto-plumbite ferrite particles.

FIG. 5 is a graph of the measured value of positive ΔM as function ofthe measured orientation ratio (OR) in tapes prepared with sphericalbarium ferrite powder and with platelet barium ferrite powder. Theplatelet and spherical particle data are from tapes prepared asexplained below. Three different platelet barium ferrite powders wereused, having the following properties: (1) particle size of about 0.8 toabout 1 μm, coercivity of about 2,880 Oe, and saturation magnetizationof about 52.4 emug; (2) particle size of about 0.5-0.7 μm and coercivityof about 1400 Oe; and (3) particle size of about 0.06-0.08 μm andcoercivity of about 980 Oe. As shown in FIG. 5, the value of ΔM formedia prepared from spherical barium ferrite powder ischaracteristically smaller than that of media prepared from plateletbarium ferrite powder, indicating the weaker strength of magneticinteraction between particles in the spherical barium ferrite powderthan in the platelet barium ferrite powder. Since the strength ofmagnetic interaction depends on the degree of stacking of particles, thelower ΔM for media prepared from spherical barium ferrite powderconfirms the expected reduction in stacking.

Controlling the substantially spherical shape of the magneto-plumbiteferrite particles is achieved in part by using a substantially sphericaliron-based oxide or hydroxide, such as magnetite (Fe₃O₄), maghemite(γ-Fe₂O₃), hematite (α-Fe₂O₃), spinel ferrite ((Co, Ni, Zn, Mn, Ti,Si)xFe_(3-x)O₄), iron hydroxide (FeOOH) or mixtures thereof as rawmaterial. Because the ionic radius of barium and strontium is similar tothat of oxygen, barium or strontium ions can diffuse into an iron-basedoxide lattice if the ions have sufficient thermal energy.

Reaction proceeds from the surface to the center of the iron oxide afterdecomposition of barium and/or strontium sources, such as carbonate(BaCO₃ and/or SrCO₃). Spherical iron-based oxide particles can becomemagneto-plumbite ferrite during calcination, while still maintaining thespherical shape of iron-based oxide. The calcination temperature isselected to be sufficiently low to ensure the desired particle shape.Control of spherical particle shape in the magneto-plumbite ferrite isthus based on the use of both spherical iron-based oxide or hydroxideand sufficiently low calcination temperature.

In the preparation process, an aqueous barium and/or strontium solutionfrom the corresponding metal halides, e.g., chlorides (BaCl₂.2H₂O orSrCl₂.6H₂O), nitrates (Ba(NO₃)₂, Sr(NO₃)₂ or Sr(NO₃)₂.4H₂O), or metalhydroxide (Ba(OH)₂.8H₂ 0 or Sr(OH)₂.8H₂O), an aqueous M chlorides and/ornitrates, an aqueous sodium or potassium carbonate ((Na₂CO₃ or K₂CO₃)solution, and an aqueous sodium or potassium hydroxide (NaOH or KOH)solution is added to a well-dispersed spherical iron-based oxide slurrywith vigorous stirring. The value for n (see formula (1) above) is inthe range of about 5.0 to about 6.0, and x, the content of substitutionelements, is in the range of about 0 to about 0.35.

The resulting mixture of spherical iron-based oxide or hydroxide, (Baand/or Sr)CO₃, and by-product, NaCl or KCl, is stirred or at least 10minutes, desirably 20 minutes, at room temperature. After mixing, thesolids of the mixed slurry are filtered out (without washing) and driedat a temperature desirably below about 150° C.

The dried particles are then calcined for a time and at a temperaturesufficient to form magneto-plumbite ferrite without significantlyaltering the substantially spherical particle shape. Such a temperatureis desirably not greater than about 900° C, with calcination generallydesirably performed in the range of about 730° C. to about 870° C. forabout 0.5 hour to about 3 hours.

After calcination, the calcined particles are wet-milled to reduceand/or prevent strong-bonding agglomeration, filtered and washed usingBuchner-type funnel. The particles are then dried and then comminuted,such as by using a pestle. The particles thus obtained are sphericalmagneto-plumbite ferrite, and have the formula given above as formula(1).

The spherical iron-based oxide or iron hydroxide such as magnetite(Fe₃O₄), maghemite (γ-Fe₂O₃), hematite (α-Fe₂O₃), spinel ferrite ((Co,Ni, Zn, Mn, Ti, Si)_(x)Fe_(3-x)O₄), and iron hydroxide (FeOOH) used asraw material should be well-dispersed to obtain both a high degree ofreaction homogeneity and stable reproducibility of magnetic properties.This may be achieved either by using a dispersion mill with strong shearstress, such as a bead mill, or by preparing a well-dispersed slurrydirectly. If the spherical iron-based oxide particles have poordispersion, the resulting calcined magneto-plumbite ferrite particlestypically have three phases such as hematite (α-Fe₂O₃), barium ironoxide (BaFeO₃ and/or BaFe₂O₄), and magneto-plumbite ferrite (AFe₁₂O₁₉).Appearance of such additional phases are attributed to agglomerates ofthe spherical iron-based oxides which prevent Ba or Sr ions fromdiffusing into the iton-based oxide particles. As a result, someiron-based oxides have deficient content of Ba or Sr ions.

Spherical iron-based oxide or hydroxide particles in the range of 0.05μm to 1.2 μm are particularly suitable for the present process. Methodsfor preparing such particles are well known. According to U.S. Pat. No.5,356,712, incorporated herein by reference, the seed crystals formed bythe initial oxidation reaction are indefinite in shape, but these seedcrystals become spherical in neutral to weakly alkaline (pH 6-9)environments. Among many factors, control of pH in the reaction is themost important factor in obtaining substantially spherical shapediron-based oxide particles. As examples, spherical iron-based oxides orhydroxide may be obtained by the procedures disclosed in U.S. Pat. No.4,992,191 (Fe₃O₄); J. Colloid and Interface Sci. 74 227-243 (1980)(Fe₃O₄); U.S. Pat. No. 5,336,421 (Zn_(x)Fe_(3-x)O₄); U.S. Pat. No.4,372,865 (iron-based hydroxide with a composition of spinel ferrite);,” J. Colloid and interface Sci. 63 509-524 (1978) (α-Fe₂O₃); J. Mater.Res. 8 2694-2701 (1993) (Fe₃O₄, and γ-Fe₂O₃, and α-Fe₂O₃); and/or J. Am.Ceram. Soc. 75 1019-1022 (1992) (Fe₃O₄ and α-Fe₂O₃). Each of theforegoing references is hereby incorporated herein by reference.

If the spherical iron-based oxide (or hydroxide) particles areover-milled, the spherical shape of the particles may be lost, with theparticles taking on an irregular or platelet shape. The milling time toobtain well-dispersed spherical iron-based oxides can vary and thereforeshould be optimized for a particular application, depending such factorsas the milling conditions including solid content, type and size of beadused, revolutions per minute (rpm) of the mill, etc.

Calcination conditions are optimized by considering both magneticproperties and dispersibility of the spherical magneto-plumbite ferriteparticles. Good spherical shape, which provides good particledispersion, requires a relatively lower calcination temperature. Optimalmagnetic properties require a relatively higher calcination temperatureto increase the crystallinity of the particles. Therefore, theparticular calcination temperature is chosen on the bases of targetedproperties of the particles. In order to decrease the calcinationtemperature to obtain good dispersibility while maintaining goodcrystallinity, flux agents and raw materials having smaller grain sizesare used. In order to substantially maintain the spherical particleshape of the precursor iron-based oxide or hydroxide particles, andthereby improve the dispersibility of magneto-plumbite ferriteparticles, the calcination temperature should generally be below about900° C. because platelet magneto-plumbite ferrite particles aretypically more stable than spherical magneto-plumbite ferrite particlesat a calcination temperatures above about 900° C.

The calcination temperature depends on a number of factors, includingparticle surface energy (particle size) as well as external thermalenergy for the diffusion of Ba or Sr into the iron oxide particles.Using sub-micron particle size BaCO₃ and/or SrCO₃ particles and a smallamount of flux agent in the form of a reaction by-product, such as KCl,NaCl, or the like, decreases the decomposition temperature of BaCO₃and/or SrCO₃. This is attributed both to the enhancement ofdecomposition of BaCO₃ and/or SrCO₃ by the flux agent and to the highsurface energy associated with smaller particle size. In addition, thecalcination temperature can be decreased by using sub-micron-sizedspherical iron-based oxide or hydroxide particles. Calcinationtemperatures and times are desirably within the ranges of from about730° C. to about 870° C. and of from about 0.5 hour to about 3 hoursrespectively, and most desirably within the ranges of from about 780° C.to about 810° C. and of from about 0.5 hour to about 2 hours. Atcalcination temperatures below about 870° C., the size and shape ofspherical magneto-plumbite ferrite particles is generally dependent onlyon that of the precursor spherical iron-based oxide (or hydroxide)particles.

The effect of the molar (Fe_(2-x)M_(x)O₃)/AO ratio (n) on the magneticproperties in the spherical magneto-plumbite ferrite is significant. Atbelow about n=5.0 in barium ferrite, (1) saturation magnetization isdecreased due to the formation of barium iron compounds such as BaFe₂O₄and BaFeO₃, and (2) coercivity distribution is very broad, whichindicates the presence of high and low coercivity phases. At above aboutn=6.0, saturation magnetization decreases to below 35 emu/g due to theformation of the hematite phase. Therefore, the value of n (the molarratio of (Fe_(2-x)M_(x)O₃)/AO) should be chosen in the range of fromabout 5.0 to about 6.0, and desirably in the range of about 5.3 to about5.8.

In order to prevent decomposition of the ultra-fine BaCO₃ and/or SrCO₃,the pH of the dispersion should be adjusted to at least about 10 orabove, preferably from about 11 to 12. To maintain the reproducibilityof desired magnetic properties, it is important to retain a givencontent of barium and/or strontium during the mixing process. If the pHof the slurry is below about 9, especially in the acid range, ultra-fineBaCo₃ and SrCO₃ can decompose, resulting in the aqueous dissolution ofBa or Sr. Such dissolution produces Ba or Sr deficiency in the oxideparticles, which results in the formation of a hematite (α-Fe₂O₃) phase.The molar (Na or K)₂CO₃/A ratio is thus desirably more than 1, and themolar (NA or K)OH/A ratio is desirably more than 2, so as to maintainthe pH above 11.

In addition to the calcination temperature, the relative amount(s) ofthe various substitution elements, selected from Co, Zn, Ni, Al, Ti, Sn,Si, Nb, Ta, and mixtures thereof, also substantially influencecoercivity, coercivity distribution, and saturation magnetization of thespherical magneto-plumbite ferrite particles. There are five distinctcrystallographic sites of sub-lattices, namely 4 f _(VI), 2 b, 12 k, 4 f_(IV), and 2 a sites, for the metallic cations in magneto-plumbiteferrite. Iron ions at these various sites make different contributionsto the magnetic properties of the magneto-plumbite ferrite. For example,the source of the uniaxial magneto-crystalline anisotropy ofmagneto-plumbite ferrite is largely the single-ion anisotropy of theiron ions at the 2 b site.

Each substitution element has a particular lattice occupation preferenceand a corresponding different contribution to magnetic properties. Forexamples, Co ions have a large magneto-crystalline anisotropy in thein-plane direction and prefer to occupy the 2 b sites with highmagneto-crystalline anisotropy in the c-axis direction ofmagneto-plumbite ferrite. The substitution of Co ions at the 2 b sitesdecreases coercivity, but increases saturation magnetization by theformation of excess spinel-block in the magneto-plumbite structure.Non-magnetic Zn ions prefer to occupy the 4 f _(IV) sites having aspin-down magnetic moment, which substitution results in an increase insaturation magnetization. Zn ion substitutions at iron sites are lesseffective in decreasing coercivity than substitutions of Co ions, butcoercivity distribution becomes narrower. Non-magnetic Ti ions prefer tooccupy the 12 k sites having a spin-up magnetic moment, whichsubstitution stabilizes the magneto-plumbite structure. Temperaturedependence of magneto-crystalline anisotropy of iron ions in 12 k sitesis larger compared to that in the other sites, so that the temperaturecoefficient of coercivity (ΔH_(c)/ΔT) of Co-Ti substitutedmagneto-plumbite ferrite reaches about +4 Oe/° C., but that in Zn-Tisubstituted magneto-plumbite ferrite is below +1.0 Oe/° C. Consequently,the substitution of Co and Ti ions significantly decreases coercivitywithout a decrease in saturation magnetization, while maintaining narrowcoercivity distribution. However, the temperature coefficient ofcoercivity (ΔH_(c)/ΔT) in Co-Ti-substituted magneto-plumbite ferrite istoo large for use in practical applications. Therefore, combinations ofsubstitution elements can be used to obtain desired or required magneticproperties. A desired combination of substitution elements comprises acombination of major substitutions of Zn-Ti with minor substitutions ofCo, Sn, and Nb or Ta.

In order to evaluate magnetic interaction among sphericalmagneto-plumbite ferrite particles in magnetic tape, hand-coated tapeswere made of magnetic paint milled for various milling times. The paintcontained 30% non-volatile materials, including spherical barium ferritepowder, an organic solvent, and a binder. Various such solvents andbinder are known in the art and may be used. For the specific examplesdescribed below, cyclohexanone was used as the solvent. The binder was avinyl chloride-copolymer binder having a functional group of SO₃ ⁻²,specifically MR 110 (obtained from Zeon Corporation, Tokyo, Japan) (20%solution).

To make the paint, 20 g of spherical barium ferrite powder are added toa solution composed of 20 g MR110 (20%) solution and 40 g ofcyclohexanone circulated at 2500 rpm in an Eiger mill. Hand-coated tapeswere prepared by known methods, using a permanent magnet with strengthof NN 3500 Gauss for particle orientation. The tapes were dried at 120°C. Tapes using platelet barium ferrite particles were prepared forcomparison by the same process.

FIG. 6 shows a side view of a magnetic recording tape such as thoseprepared in the examples below. A substrate 20, typically non-magnetic,supports a magnetic layer 22 that formed by coating a magnetic paintonto the substrate 20. Suitable substrates are well known in the art andinclude various polymers such as PET (polyethylene terephthalate) andother film-forming materials. The present invention is also applicableto magnetic media such as shown in FIG. 7, wherein magnetic particlesare dispersed within a self-supporting body 24, such as a magnetic card.The self-supporting body 24 may be formed of any suitable material, suchas a polymer, in which the magnetic particles are dispersed. Asexamples, PVC (polyvinyl chloride) is currently used for credit cards,while PETG (a recyclable copolyester) has been suggested as a moreenvironmentally acceptable substitute.

Magnetic properties of hand-coated tapes prepared according to theinvention were measured with a vibrating sample magnetometer (VSM) fromDigital Measurement Systems Division of ADE Technologies, BurlingtonMass., USA.

Specific examples of the process of the invention and the resultinginventive particles are given below. These examples describe particularfeatures of working embodiments and should not be construed to limit theinvention to the particular features described.

EXAMPLE 1

A dispersion comprising (1) 30 g of 08 μm spherical Fe₃O₄ and (2) 150 mlof a solution of 8.34 g of BaCl₂. 2H₂O, 1.47 g of ZnCl₂, and 2.04 g ofTiCl₄ was prepared using an Eiger mill, which is a type of bead mill. Asolution of 4.11 g of NaOH and 5.43 g of Na₂CO₃ in 60 ml of water wasthen added to the vigorously stirred solution. The slurry thus obtainedwas kept at room temperature for 20 minutes, while stirring.

After mixing, the solid phase of the slurry was filtered off using aBuchner-type funnel with a vacuum pump, and dried in a fume hood for 12hours. The resulting dry material was dry-milled by mortar and pestle.The resulting comminuted powder was then calcined at 810° C. for 2hours. The calcined powder was then wet-milled using an Eiger mill,washed to remove sodium and chloride ions, and dried at 130° C. for 6hours.

The resulting powder consisted of a single BaFe₁₂O₁₉ phase, according tox-ray diffraction patterns. FIG. 8 is a plot of the peaks of the X-raydiffraction analysis of this powder. TEM observation showed the powderwas composed of substantially spherical particles having a mean particlediameter of 0.9 μm. The particles' coercivity (H_(c)) was 2,750 Oe, andthe specific saturation magnetization was (σ_(S)) 49.4 emu/g. Themagnetic properties of hand-coated tape made using magnetic paint milledfor 4 hours were measured as follows: coercivity 2,617 Oe, squarenessratio (SQ) 0.71, switching field distribution (SFD) 0.30, orientationratio (OR) 1.64, and value of positive ΔM 0.16.

EXAMPLE 2

Spherical barium ferrite powder was prepared in the same process as inExample 1 except that spherical Fe₃O₄ having a particle size of 0.2 μmwas used as the spherical iron-based oxide (or hydroxide) startingparticles.

The resulting powder consisted of major BaFe₁₂O₁₉ and minor BaFeO₃phases according to the x-ray diffraction patterns. A graph of the X-raydiffraction peaks for this powder is shown in FIG. 9. The TEM micrographin FIG. 2 shows the resulting substantially spherical 0.2 μm-size bariumferrite particles. The coercivity (H_(c)) of the produced sphericalpowder was 1,474 Oe, and the specific saturation magnetization (σ_(s))was 40 emu/g.

The magnetic properties of hand-coated tape, made using magnetic paintmilled for 4 hours, were as follows: coercivity 1,583 Oe, squarenessratio (SQ) 0.67, switching field distribution (SFD) 0.37, orientationratio (OR) 1.40, and value of positive ΔM 0.10. This small value ofpositive ΔM, shown in FIG. 2, indicates the weaker strength of magneticinteractions between particles in the spherical barium ferrite tape thanthe platelet barium ferrite tape.

EXAMPLE 3

A 80 g dispersion containing 12.5 wt. % of well-dispersed 0.8 μmspherical Fe ₃O₄ particles was prepared directly by aging an Fe- and Ti-ion-containing solution at 96° C. for 4 hours and washing. Both asolution of 2.78 g of BaCl_(2.)2H₂O in 20 ml of water and a solution of1.37 g of NaOH and 1.81 g of Na₂CO₃ in 30 ml of water were added intothe vigorously stirred slurry of the above dispersion with continuousvigorous stirring. The slurry thus obtained was kept at room temperaturefor 20 minutes, while stirring. The solid phase of the dispersion wasthen filtered off using a Buchner-type funnel with vacuum pump, anddried in a fume hood for 12 hours. The resulting dry material was thencomminuted by dry-milling by mortar and pestle. The thus comminutedpowder was then calcined at 800° C. for 2 hours. The calcined powder waswet-milled using an Eiger mill. After wet-milling, the powder was washedwith water to remove sodium and chloride ions, and dried at 130° C. for6 hours.

The resulting powder consisted of a single BaFe₁₂O₁₉ phase, according tox-ray diffraction patterns, and was composed of spherical particleshaving a particle size of 0.8 μm. This single phase barium ferritepowder had a coercivity (H_(c)) of 4,335 Oe and specific saturationmagnetization (σ_(s)) 49 of emu/g.

EXAMPLE 4

Spherical strontium ferrite powder was obtained by the same process asdescribed in Example 3 except a solution of 3.04 g of SrCl₂.6H₂O wasused instead of BaCl₂.2H₂O. The resulting powder consisted of a singleSrFe₁₂O₁₉ phase, according to x-ray diffraction patterns, and wascomposed of spherical particles having a particle size of 0.8 μmCoercivity (H_(c)) was 5,016 Oe, and specific saturation magnetization(σ_(S)) was 45.2 emu/g.

EXAMPLE 5

A dispersion was prepared using an Eiger mill. The dispersion was formedby combining 16.66 g of 0.4 μm-sized spherical α-Fe₂O₃ and 100 ml of anaqueous solution comprising 5.38 g of BaCl₂.2H₂O, 0.98 g of ZnCl₂, and1.32 g of TiCl₄. A solution of 2.74 g of NaOH and 3.62 g of Na₂CO₃ in 50ml of water was then added into the above vigorously stirred dispersion.The resulting slurry was kept at room temperature for 20 minutes, whilestirring. After stirring, the solid phase of the dispersion was filteredoff using a Buchner-type funnel with a vacuum pump, and was dried in afume hood for 12 hours. The resulting dried material was dry-milled witha pestle. The thus comminuted powder was calcined at 800° C. for 1 hour.The calcined powder was wet-milled using an Eiger mill, washed withreverse-osmosis (RO) water to remove sodium and chloride ions, and driedat 130° C. for 6 hours.

The resulting powder consisted of major BaFe₁₂O₁₉ and minor BaFeO₃phases, according to x-ray diffraction patterns. The thus obtainedspherical barium ferrite particles had a mean particle size of 0.4 μm,coercivity of 1,720 Oe, and specific saturation magnetization of 41.2emu/g. The particles thus obtained are shown in the micrograph of FIG.1.

EXAMPLE 6

A dispersion was prepared using an Eiger mill. The dispersion was formedby combining 10 g of 0.1 μm-size substantially spherical Fe₃O₄,including Ti (Ti/Fe=9 mol %) ions, and 80 ml of an aqueous solutioncomprising 2.78 g of BaCl₂.2H₂O and 0.49 g of ZnCl₂. A solution of 1.37g of NaOH and 1.81 g of Na₂CO₃ in 30 ml of water was then added to theabove vigorously stirred dispersion. The slurry obtained was kept atroom temperature for 20 minutes, while stirring. The solid phase of thedispersion was then filtered off using a Buchner-type funnel with vacuumpump, and was dried in a hood for 12 hours. The resulting dry materialwas dry-milled by mortar and pestle. The comminuted powder was thencalcined at 780° C. for 2 hours. The calcined powder was wet-milledusing an Eiger mill, washed with reverse-osmosis (RO) water to removesodium and chloride ions, and dried at 130° C. for 6 hours. Theresulting powder consisted of major BaFe₁₂O₁₉ and minor BaFeO₃ phases,according to x-ray diffraction patterns. The thus-obtained sphericalbarium ferrite particles had a particle size of 0.1 μm, a coercivity of1,367 Oe, and a specific saturation magnetization of 38.2 emu/g.

EXAMPLE 7

A dispersion was prepared using an Eiger mill. The dispersion was formedby combining 10 g of 0.1 μ m spherical Fe₃O₄, including Ti ions (Ti/Fe=9mol %), and 80 ml of an aqueous solution comprising 4.29 g ofBa(OH)₂.8H₂O and 0.62 g of ZnCl₂. A solution of 5.00 g of Na₂CO₃ in 30ml of water was then added to the above vigorously stirred dispersion.The slurry thus obtained was stirred at room temperature for 20 minutes.After mixing, the solid phase of the dispersion was filtered off usingBuchner-type funnel with vacuum pump, and was dried in a hood for 12hours. The resulting dried material was dry-milled by mortar and pestle.The resulting comminuted powder was calcined at 790° C. for 1.5 hours.The calcined powder was then wet-milled using an Eiger mill, washed withreverse-osmosis (RO) water to remove sodium and chloride ions, and driedat 130° C. for 6 hours.

The resulting powder consisted of major BaFe₁₂O₁₉ and minor BaFeO₃phases according to x-ray diffraction patterns. The thus-obtainedspherical barium ferrite particles had a particle size of 0.1 μm,coercivity of 1,315 Oe, and specific saturation magnetization of 40.4emu/g.

In view of the many possible implementations of the invention, it shouldbe recognized that the specific implementations above are only examplesof the invention and should not be taken as a limitation on the scope ofthe invention. Rather, the scope of the invention is defined by thefollowing claims. We therefore claim as our invention all that comeswithin the scope and spirit of these claims.

We claim:
 1. A process for the preparation of a sphericalmagneto-plumbite ferrite powder having the formulaAO.n(Fe_(2-x)M_(x)O₃), wherein A is at least one of barium andstrontium, n is the molar (Fe_(2-x)M_(x)O₃)/AO ratio in the range ofabout 5 to about 6, M is more than one substitution element selectedfrom the group of Co, Zn, Ni, Al, Ti, Sn, Si, Nb, Ta, and mixturesthereof, and x is the content of substitution elements in the range ofabout 0 to about 3.5, the process comprising: (a) mixing a dispersion ofprecursor substantially spherical particles, of one or more ofiron-based oxide and iron hydroxide, with a first solution of one ormore of barium and strontium, and with a second solution of one or moreof Na₂CO₃, K₂CO₃, NaOH, and KOH; (b) separating a solid phase from thedispersion; (c) drying the separated solid phase; and (d) calcining thedried separated solid phase at a temperature sufficiently low to producemagneto-plumbite particles from the precursor particles while preservingthe substantially spherical shape of the precursor particles.
 2. Theprocess recited in claim 1 further comprising the steps of: (e)wet-milling the calcined powder; (f) drying the wet-milled powder; and(g) comminuting the dried powder.
 3. The process recited in claim 2further comprising washing the calcined powder.
 4. The process recitedin claim 1 further comprising comminuting the dried separated solidphase before calcining.
 5. The process recited in claim 1 wherein thestep of calcining the comminuted powder comprises calcining thecomminuted powder at a temperature not more than about 900° C.
 6. Theprocess recited in claim 5 wherein the step of calcining the comminutedpowder comprises calcining the comminuted powder at a temperaturesufficient to convert the precursor particles into magneto-plumbiteferrite while substantially maintaining the shape of the precursorparticles.
 7. The process recited in claim 6 wherein the step ofcalcining the comminuted powder comprises calcining the comminutedpowder for a time in the range of about 0.5 to about 3 hours.
 8. Theprocess recited in claim 6 wherein the step of calcining thecommuninuted powder comprises calcining the communinuted powder at atemperature in the range of about 730 to about 870° C.
 9. The processrecited in claim 8 wherein the step of calcining the comminuted powdercomprises calcining the comminuted powder for a time in the range ofabout 0.5 to about 2 hours.
 10. The process recited in claim 1 whereinthe precursor particles comprise one or more of magnetite, maghemite,hematite, spinel ferrite, and iron hydroxide.
 11. The process recited inclaim 1 wherein the precursor particles have a particle size in therange of about 0.05 to about 1.2 μm.
 12. The process recited in claim 11wherein the precursor particles have a particle size of less than about0.2 μm.
 13. The process recited in claim 1 wherein the solution of oneor more of Ba and Sr is formed from one or more of barium chloride,barium nitrate, barium hydroxide, strontium chloride, strontium nitrate,and strontium hydroxide.
 14. The process recited in claim 13 wherein apowder is mixed with a solute to form the solution of one or more of Baand Sr, the powder consisting of particles of submicron size.
 15. Theprocess recited in claim 1 wherein the step of mixing comprises mixingin a bead mill for at least about 10 minutes.
 16. The process recited inclaim 1 wherein the substitution elements consist of a majorsubstitution of Zn and Ti and a minor substitution of Co, Sn, and atleast one of Nb and Ta.
 17. The process recited in claim 1 wherein n isin the range of about 5.3 to about 5.8.
 18. The process recited in claim1 wherein, in the step of mixing, the amount of the one or more ofNa₂CO₃, K₂CO₃, NaOH, and KOH in the second solution is such that theentire mixture of precursor particles and first and second solutions hasa pH in the range of about 10 or greater.
 19. The process recited inclaim 18 wherein, in the step of mixing, the amount of the one or moreof Na₂CO₃, K₂CO₃, NaOH, and KOH in the second solution is such the pHduring mixing is in the range of about 11 or greater.
 20. The processrecited in claim 1 wherein, in the step of mixing, the molar (Na orK)₂CO₃/A ratio is more than 1, and the molar (NA or K)OH/A ratio is morethan 2.